Natural killer (NK) cells are cytolytic lymphocytes, distinct from B-lymphocytes and T-lymphocytes, that participate in both innate immunity and adaptive immunity. NK cells lack B-cell receptors and T-cell receptors. NK cells play a key role in the elimination of tumor cells or negative major histocompatibility complex (MHC) class I cell variants. NK cells generally kill cells by releasing pore-forming proteins called perforins, proteolytic enzymes called granzymes, and chemokines. NK cells appear to use a dual receptor system in determining whether to kill or not kill potential target cells. Hence, NK cells play an important role in nonspecific anti-tumor immunity and prevent the establishment of primitive or metastatic tumors in both immunocompetent and immunosuppressed hosts. In particular, the role of NK cells in anti-tumor immunosurveillance has been suggested in mice (see, U.S. Pat. No. 6,849,452).
Because of their non-specific cytotoxic properties for antigen and their efficacy, NK cells constitute an important population of cells for the development of immunoadoptive approaches to the treatment of cancer or infectious diseases. Anti-tumor adoptive immunotherapy approaches have been described in the literature. Thus, in certain situations, for example, patients with malign lymphomas, the results of administering adherent NK cells with small doses of interleukin-2 (IL-2) have been promising in an adjuvant situation. NK cells have also been used for experimental treatment of different types of tumors and certain clinical studies have been initiated (Kuppen et al., 1994; Lister et al., 1995; Rosenberg et al., 1987).
Further, such cells can also be used in vitro and in vivo for lysis of cells which do not express MHC class I molecules, and more generally any cell that is sensitive to NK cells.
Adaptable therapies using NK cells (for example, treatment of human tumors or infectious diseases) or any other in vitro or in vivo use of NK cells frequently involves ex vivo expansion and activation of the NK cells.
Notch receptors and DSL (Delta-Serrate-Lag2) ligands assist in cell fate decisions during embryogenesis (Simpson et al., 1995; Robey et al., 1997; Lewis et al., 1998; Artavanis-Tsakonas et al., 1999; and Tax et al., 1994). Four Notch receptors (Notch-1, -2, -3 and -4) (del Amo et al., 1993; Weinmaster et al., 1992; Lardelli et al., 1994; and Uyttendaele et al., 1996) and five DSL ligands (Jagged1, Jagged2, Delta-like-1 or Dll-1, Dll-3 and Dll-4) (Lindsell et al., 1995; Shawber et al., 1996; Tsai et al., 2000; Bettenhausen et al., 1995; Dunwoodie et al., 1997; and Shutter et al., 2000) have been identified in the murine system. After birth, mice continue to express Notch receptors and DSL ligands in many tissues. However, little is known about their functions in adult mice. Recently, reverse genetics and cell culture studies have begun to shed light on the functional roles of Notch receptors and DSL ligands in T and B cell development (Pui et al., 1999; Radtke et al., 1999; Tanigaki et al., 2002; Radtke et al., 2004; Schmitt et al., 2002; and Schmitt et al., 2004). Conditional knockout of Notch-1 in the postnatal period abolishes T cell development (Radtke et al., 1999). The thymuses of these knockout mice also lack a well-developed cortex.
In tissue culture, Lin− Sca-1+ c-Kit+ murine hematopoietic stem cells (HSC) stimulated by Flt3 ligand (Flt3L), interleukin-7 (IL-7) and OP-9 stromal fibroblasts expressing ectopic Dll-1 undergo de novo T cell development (Schmitt et al., 2002) (see also, U.S. Patent Application No. 2004/0171148). Similar findings were made using progenitors derived from embryonic stem cells (ESC) (Schmitt et al., 2004). Subsequent studies indicate that continued presence of Dll-1 is required for T cell commitment and maintenance at the double-negative 1 (DN1) and 2 (DN2) stages of thymocyte development. In its absence, the developing DN1 and DN2 thymocytes adopt the NK cell fate by default (Schmitt et al., 2004). More recently, it was reported that a conditional knockout of Dll-1 blocks the development of marginal B cells, but has no effect on T cell development (Hozumi et al., 2004). This observation seems to contradict the findings of the Dll-1 studies (Schmitt et al., 2002; Schmitt et al., 2004). Nonetheless, the results of these studies underscore the importance of DSL ligands in lymphopoiesis.